The present invention relates to galvanized steel sheets intended for use in applications such as automobiles, electric household appliances and building materials and having improved characteristics including corrosion resistance and workability.
Traditionally, galvanized steel sheets for use in applications such as automobiles, electric household appliances, and building materials are in many cases subjected to phosphate treatment, chromate treatment and organic coating treatment prior to their use in order to impart properties such as corrosion resistance and workability and thereby improve their value. In view of the effect that they may impose to the environment, the chromate-treated steel sheets have become less favored as they may contain chromium (VI), and demands are increasing for phosphate treatments. Znxe2x80x94Ni alloy-plated steel sheets are widely in use because of their high corrosion resistance and workability. However, containing Ni, the alloy plating leads to an increased manufacturing costs. For these reasons, attempts have been made to apply phosphate treatments to less costly electrogalvanized steel sheets, hot-dip galvanized steel sheets, or alloy hot-dip galvanized steel sheets to thereby improve their value.
However, when electrogalvanized steel sheets, hot-dip galvanized steel sheets, or alloy hot-dip galvanized steel sheets are subjected to phosphate treatments in a conventional manner, the resulting workability is not as significant as that of Znxe2x80x94Ni alloy-plated steel sheets. In addition, these steel sheets exhibit a lower corrosion resistance as compared to organic composite steel sheets, which are formed by depositing chromate layer and an organic layer over Znxe2x80x94Ni alloy plating. Steel sheets including a phosphate layer and an organic layer formed over the phosphate layer are also known. These steel sheets have not been put to practical use, however, since the organic layer tends to become too thick if sufficient corrosion resistance is pursued. These steel sheets are also associated with problems concerning weldability, workability, and costs.
Accordingly, it is an objective of the present invention to provide a galvanized steel plate that has overcome the above-identified problems and has its characteristics, such as corrosion resistance and workability, improved without being subjected to chromate treatments.
In an effort to improve performances of the phosphate-treated galvanized steel sheets, including corrosion resistance and workability, the present inventors have examined the possibility of forming an additional organic layer on top of a phosphate layer. However, it has tuned out that any combination of conventional treatments cannot provide sufficient adhesion between the phosphate layer and the organic layer and is likely to result in formation of blisters upon electrodeposition of the coating layer. Accordingly, no significant improvement is expected in the corrosion resistance or the workability. In the course of their studies to find a way to solve these problems, the present inventors have discovered that highly favorable characteristics are achieved by treating a surface of a galvanized steel sheet with zinc phosphate that contains Mg as an essential component and by forming an organic layer on top of the zinc phosphate layer, and have thereby completed the present invention.
In one aspect, the present invention provides an organic composite galvanized steel sheet formed by sequentially depositing on at least one surface of a steel sheet, a galvanization layer, a zinc phosphate layer in an amount of 0.3 g/m2 or more, and an organic layer in an amount of 0.3 to 2 g/m2. The organic composite galvanized steel sheet is characterized in that the zinc phosphate layer contains Mg so that the value of Mg/P (weight ratio) in the zinc phosphate layer is 0.15 or larger and the amount of Mg contained in the zinc phosphate layer is 20 mg/m2 or more. Preferably, the zinc phosphate layer contains one or two or more of Ni, Mn, Co, Fe, Cu, Al, and Ca. In one preferred embodiment, the organic layer is formed as a composite layer containing an organic resin and one or two kinds or more of powder or colloid selected from SiO2, Al2O3, MgO, Fe2O3, Fe3O4, ZrO2, TiO2, and SnO2.
The galvanization for use in the present invention is not limited to a particular type and may be pure zinc galvanization or alloy galvanization. In either case, significant improvements in the corrosion resistance and workability can be realized. However, electrogalvanization, hot-dip galvanization, and alloy hot-dip galvanization are preferred in view of manufacturing costs. Also, the galvanization may be applied as a single-layered plating or a multi-layered plating, or it may be a galvanization layer deposited on a pre-plating of Ni, Cu and the like.
It is essential that the zinc phosphate layer deposited on the galvanization layer contain Mg. This constitutes one of the key features of the present invention. The amount of Mg is at least 0.15 as measured in the value of Mg/P (weight ratio). If this value is smaller than 0.15, the corrosion resistance is not improved. The upper limit of this value is typically about 0.78 though not particularly limited, and it is difficult to have the zinc phosphate layer contain Mg in larger amounts. By forming a zinc phosphate layer containing Mg in the aforementioned ratio, adhesion to the organic layer deposited on top of the zinc phosphate layer is significantly improved and formation of blisters upon electrodeposition of the layer is effectively prevented. Although underlying mechanisms for these phenomena have not been fully understood, it is guessed that the presence of Mg enhances the flexibility of the zinc phosphate layer to prevent occurrence of cohesive failure in the zinc phosphate layer and reduces its solubility under basic environments. Mg is also known to have a significant ability to stabilize corroded products of Zn and thus suppresses progress of corrosion, contributing to improvements in the corrosion resistance.
The amount of Mg in the zinc phosphate layer needs to be 20 mg/m2 or more in order to achieve a high corrosion resistance. Aside from Mg, the zinc phosphate layer preferably contains one or two or more of Ni, Mn, Co, Fe, Cu, Al, and Ca. This further improves the corrosion resistance and workability.
The zinc phosphate layer has a weight of 0.3 g/m2 or more, preferably in a range from 0.3 g/m2 to 2 g/m2. The weight less than the lower limit of this range may lead to an insufficient corrosion resistance, whereas the weight exceeding the upper limit makes the layer susceptible to peeling when it is subjected to strict working processes.
A preferred treatment solution for forming the zinc phosphate layer is prepared as a bath by adding a large amount of magnesium nitrate to a commercially available treatment solution containing Zn ions and phosphate ions as principal components and optionally containing metal ions other than Zn, nitrate ions, fluorides and the like. The amount of Mg and the ratio of Mg to P in the layer can be controlled by adjusting the amount of magnesium nitrate to be added, and the coating amount of the zinc phosphate layer can be adjusted by varying treatment time.
According to the present invention, sufficient performances are achieved without using the treatment known as chromate sealing, which generally follows the zinc phosphate treatment.
With regard to the organic layer, it may be composed solely of an organic resin while it is preferably formed as a composite layer composed of an organic resin, and one or two kinds or more of powder or colloid selected from SiO2, Al2O3, MgO, Fe2O3, Fe3O4, ZrO2, TiO2, and SnO2 in view of corrosion resistance. The organic layer may also contain a wax component, a color pigment, a rust-preventing agent and the like. The wax component permits working without applying oil depending on conditions of the shaping processes. Considering the fact that at present epoxy coatings are widely in use as an electrodeposition coating in automobile steel sheet application, an epoxy resin is preferred because of its compatibility with the electrodeposition coating and its ability to ensure close adherence, while no particular limitation is imposed on the type of the organic resin.
The coating amount of the organic layer is in a range of 0.3 g/m2 to 2 g/m2. The coating amount less than the lower limit of the range results in insufficient improvements in the workability, whereas the coating amount exceeding the upper limit of the specified range leads to a reduced weldability. When applied to outer plates of automobiles, the organic layer is applied there so that the coating amount is different on each side of the plate, for example, less than 0.3 g/m2, or preferably zero, on the outer side of the plate and 0.3 g/m2 to 2 g/m2 on the inner side of the plate, since the organic layer makes the electrodeposition appearance of the outer side of the outer plates unattractive when applied in excessive amounts. In this manner, most well-balanced characteristics are achieved.
When intended for use as outer plates of automobiles, the organic composite galvanized steel sheet preferably includes layers with the below-described constructions deposited on the inner side and on the outer side of the plate, respectively.
(Inner Side)
A galvanization layer, a zinc phosphate layer in an amount of 0.3 g/m2 or more, and an organic layer in an amount of 0.3 g/m2 to 2 g/m2 are sequentially deposited. The zinc phosphate layer contains Mg so that the value of Mg/P (weight ratio) is 0.15 or larger and the amount of Mg is 20 mg/m2 or more in the zinc phosphate layer.
(Outer Side)
A galvanization layer, a zinc phosphate layer in an amount of 0.3 g/m2 or more, and an organic layer in an amount of 0.3 g/m2 or less are sequentially deposited. The zinc phosphate layer contains Mg so that the weight ratio of Mg to P is 0.15 or larger and the amount of Mg is 20 mg/m2 or more in the zinc phosphate layer. The organic layer may be 0 g/m2, or not at all provided, on the outer side.